1. Field of the Invention
This invention relates to a method of controlling a tension which acts on a workpiece fed in between the stands of a tandem rolling mill. More particularly, it relates to a method of selective control between a tension control method with a looper and a looperless tension control method.
During the threading stage of the top end and during the tailout stage, it is difficult to precisely control the tension by means of the looper, and hence, the looperless tension control is made. Under the steady rolling state, the tension control employing the looper is made.
2. Description of the Prior Art
In a broad sense, this invention concerns the control of a tension between respective stands in a tandem rolling mill.
In the tandem rolling mill in which the adjacent stands are coupled by the tension of a strip, when the final thickness of the strip produced is to be held constant, each interstand tension should be controlled into a substantially constant value during the operation of the rolling mill so that the material of an identical volume may pass every predetermined unit time.
For this tension control, the two control methods of the so-called looperless control method and the control method with a looper have been tried.
First, the looperless tension control method will be explained. U.S. Pat. No. 3,940,960 entitled "Interstand Tension Control Method and Apparatus for Tandem Rolling Mills" (Mar. 2, 1976, by S. Tanifuji et al) has been issued to the same applicants as in the present application, and its contents are as stated below.
In a method of controlling a tension which acts on a workpiece threaded between rolling stands of a tandem rolling mill constructed of at least a first stand and a second stand, the patented invention is fundamentally characterized by comprising the step of calculating a reference torque arm of said first stand in a period from the threading of said workpiece into said first stand to the threading thereof into said second stand, the step of detecting torque arms of said first and second stands at a time immediately after the threading of said workpiece into said second stand, and calculating a reference torque arm of said second stand from the detected torque arms of said first and second stands and said reference torque arm of said first stand and then storing the calculated result, and the step of thereafter detecting torque arms of said first and second stands during rolling (under the state under which said workpiece is threaded into both said first and second stands), calculating a difference between a variation of the detected first-stand torque arm value from the first-stand reference torque arm and a variation of the detected second-stand torque arm value relative to the second-stand reference torque arm, and controlling the tension of said workpiece between said first and second stands into a constant value according to the calculated difference value.
This expedient determines the reference torque arm of each stand with the torque arms of the ajacent stands immediately after the threading of each stand and the torque arm of the preceding stand at no tension, not with the torque arm immediately after the threading of each stand, and therefore has such an effect that the error of a detected value ascribable to an impact drop immediately after the threading is little.
On the other hand, the tension control method with a looper is disclosed in, for example, U.S. Pat. No. 3,332,263 entitled "Computer Control System for Metals Rolling Mill" (July 25, 1967, by R. G. Beadle et al).
This concerns the automatic operation of a hot steel-plate rolling mill by the instructions of a controlling computer. The computer stores an operating program, and besides, it receives input data and external control data and combines them with the stored program so as to sequentially renew the program.
FIGS. 2A-2D of the drawings of the cited patent show schematic views of a rolling mill, and Column 5, line 55--Column 6, line 24 of the specification thereof refer especially to the control of the looper.
As set forth above, there are the two methods of controlling the tension without and with the looper. In the technical field of the rolling mill control employing a digital computer, the looperless control method is being adopted increasingly. However, any literature or report stating merits owing to the joint use with the looper control is not found.